The invention relates to a display device comprising a display screen which is provided with a light-absorbing coating of an inorganic network comprising at least silicon oxide and a dyestuff.
The invention also relates to a method of manufacturing a light-absorbing coating of an inorganic network comprising at least silicon oxide on a display screen of a display device by means of a sol-gel process, an aqueous solution of an alkoxysilane and a dyestuff being provided on the display screen and heated to convert the solution into said coating.
Light-absorbing coatings which are used to reduce the light transmission are applied to the face plates of display devices, such as cathode ray tubes (CRTs), liquid crystal display devices (LCD and LC-TV) and thin electron displays, to improve the contrast of the image reproduced. By virtue thereof, there is no necessity to change the glass composition of the display screen and the possibilities of bringing the light transmission to a desired value in a simple manner are increased. Such coatings reduce the transmission of both incident ambient light and light originating from CRT phosphors or colour filters. As the coatings can be provided in a uniform manner, the transmission of the filtering layers is also uniform. The incident ambient light passes through the coating and the glass display screen, after which it reflects off, respectively, the rough phosphor layer or the colour filter on the inside of the display screen and again passes through the display screen and the coating. If the transmission of the coating is designated T, the intensity of the reflected ambient light is reduced by a factor of T.sup.2. Light originating from, respectively, the phosphors and the colour filter passes through the coating only once, so that the intensity of this light is reduced by a factor of T only, resulting in an increase of the contrast by a factor of T. There are transmission or T-coatings the absorption of which is substantially independent of the wavelength of the visible light and which, as a result, are of a neutral-grey colour, and chrominance or C-coatings which selectively absorb one or more spectral regions of the visible light. In the latter case, the absorption is preferably selected so that it takes place in the spectral region situated between the emission spectra of the phosphors.
In U.S. Pat. No. 4,987,338 a description is given of a cathode ray tube having a display screen which is provided with a coating comprising a silicon (di)oxide network and a dyestuff having a selective light absorption which is maximal in the wavelength range of 575.+-.20 nm. Said Patent Specification also describes a method of manufacturing such a layer by means of a sol-gel process in which an aqueous alcoholic solution of an alkoxysilane compound, such as tetraethyl orthosilicate Si(OC.sub.2 H.sub.5).sub.4 (TEOS), acidified with hydrochloric acid and to which a dyestuff is added, is spin coated onto a display screen. For example, Rhodamine B is used as the dyestuff. Silicon (di)oxide starts forming in the solution. By a treatment at an increased temperature, the formation of silicon (di)oxide is completed and a layer of a silicon (di)oxide network which also contains the dyestuff is formed.
In the non-prepublished European Patent Application EP-A-603941, filed by Applicants, a description is given of a method of manufacturing a neutral, absorbing coating of silicon oxide, metal oxide and a black dyestuff by means of a sol-gel process, said coating having an improved light fastness and chemical resistance.
A disadvantage of the known method is that the maximally attainable layer thickness of the sol-gel coating is 0.8 .mu.m owing to the large quantities of water and alcohol to be evaporated, which leads to shrinkage during the curing operation. Due thereto, the risk of crackles in the layer increases as the layer thickness increases. The maximum concentration of dyestuff in a sol-gel layer is limited. Larger quantities of dyestuff cause the chemical resistance and the mechanical properties of the layer, such as hardness and resistance to wear, to be reduced. This is caused by the fact that the silicon-oxide network is disturbed, which has the additional disadvantage that the dyestuff can be washed out. Owing to the small layer thickness of the known sol-gel layers, the maximum quantity of dyestuff in the layer is limited, so that the maximally attainable light absorption of the layer is limited too. This can be compensated for by means of a second sol-gel layer comprising dyestuff. However, this requires an additional process step.
The known, relatively thin sol-gel layers additionally require a smooth, high-polish display-screen surface in order to obtain a coating having a high-polish surface. This means that the average roughness R.sub.a across the substrate surface must not exceed 0.05 .mu.m and that the maximum roughness R.sub.z must not exceed 0.3 .mu.m. For this purpose, the display-screen surface must be subjected to an expensive fine-polishing treatment, for example with Ce.sub.2 O.sub.3.